U.S. Pat. No. 3,496,954 discloses an altitude sensor for operating an oxygen regulator which supplies a recipient with oxygen enriched air up to a predetermined altitude and thereafter pure oxygen to maintain normal physiological functions. However, it was found that by tying an oxygen flow proportioning control valve to an air flow valve, as shown in U.S. Pat. No. 3,509,895, it was possible to provide a fixed proportional relationship for the mixing of oxygen and air. It was later determined that a check valve was required to assure that oxygen was not communicated to the atmosphere during periods of expiration by the recipient. Unfortunately, such check valves were not readily accessible for maintenance or adjustment.